Conventionally, antenna devices for radar narrow down an electromagnetic wave, which is radiated so as to be vertically spread into a beam shape using a metal horn. This configuration is disclosed in JP2005-73212(A), for example.
However, in order to obtain a desired directivity with the metal horn, it is necessary to extend a projecting length of the horn in the radiating direction of the electromagnetic wave, or to expand an aperture angle. As a result, the entire antenna device is increased in size.